(determined by ligand binding) differed between the two groups
(mean 111 and 60, respectively), not all luminal A tumors showed
high values (9 ⬎ 100 fmol兾mg; 5, 30–100 fmol兾mg; 4, 10–30
fmol兾mg; 1 ⬍ 10 fmol兾mg). It also should be noted that the ER⫹
protein category cases based on ligand binding were highly
heterogeneous with respect to their gene expression profiles
(18兾19 were in luminal A, 5兾5 in luminal B, 9兾10 in luminal C,
2兾7 in basal-like, 4兾5 in ERBB2⫹, and 3兾5 in normal breast-like
tumors). The luminal subtype B ⫹ C tumors might represent a
clinically distinct group with a different and worse disease
course, in particular with respect to relapse (Fig. 3 A and B).
Luminal subtype C was associated with the worst outcome of the
three presumed subtypes when a six-subtype classification
formed the basis for the survival analysis (Fig. 3C). The potential
clinical significance of this molecular subtype is highlighted by
the similarities in expression of some of the genes that are
characteristic of the ER-negative tumors in the basal-like and
ERBB2⫹ subtypes (Fig. 1D), which suggests that the high level
of expression of this set of genes is associated with poor disease
outcomes. The difference in outcome between the different
subgroups was confirmed in a subanalysis based on the five
subgroups identified when using the intrinsic gene list and only
the 51 Norway carcinomas (Figs. 2 and 5), as seen in Fig. 3D.
The genomewide expression patterns of tumors are a repre-
sentation of the biology of the tumors; the diversity in patterns
reflects biological diversity. Thus, relating gene expression pat-
terns to clinical outcomes is a key issue in understanding this
diversity. Although many parameters have been explored in
relation to breast cancer biology and outcome, the finding that
patients with tumors expressing the ER have a relatively favor-
able prognosis, despite the fact that estradiol is a highly potent
mitogen in receptor-positive cells, underlines the problems of
correlating different parameters and extrapolating knowledge
about the biological function of a single factor from its prog-
nostic value. The ability to classify tumors into distinct entities
by identifying recurrent gene expression patterns of hundreds or
thousands of genes would further enable identification of com-
binations of marker genes that otherwise would be unrecognized
by standard methods and help to get a deeper understanding of
the function of gene interplay. In this article we have provided
evidence for a relationship between five expression-based sub-
classes of breast tumors and patient outcome. Of particular
interest is the finding that ER⫹ tumors may be subclassified into
distinct subgroups with different outcomes. Furthermore, these
studies set the stage for a larger and more elaborate study in
which many additional breast tumors need to be examined and
combined with detailed clinical information, which then will
provide a means for identifying expression motifs that represent
important clinical phenotypes, like resistance and sensitivity to
specific therapies, invasiveness, or metastatic potential.
We are grateful to the National Cancer Institute, the Norwegian
Research Council, the Norwegian Cancer Society, and the Howard
Hughes Medical Institute who provided support for this research. T.S. is
a postdoctoral fellow of the Norwegian Cancer Society. P.O.B. is an
Associate Investigator of the Howard Hughes Medical Institute.
1. Fisher, E. R., Costantino, J., Fisher, B. & Redmond, C. (1993) Cancer 71,
2. Elston, C. W. & Ellis, I. O. (1991) Histopathology 19, 403–410.
3. Torregrosa, D., Bolufer, P., Lluch, A., Lopez, J. A., Barragan, E., Ruiz, A.,
Guillem, V., Munarriz, B. & Garcia Conde, J. (1997) Clin. Chim. Acta 262,
4. Vollenweider-Zerargui, L., Barrelet, L., Wong, Y., Lemarchand-Beraud, T. &
Gomez, F. (1986) Cancer 57, 1171–1180.
5. Foekens, J. A., Look, M. P., Bolt-de Vries, J., Meijer-van Gelder, M. E., van
Putten, W. L. & Klijn, J. G. (1999) Br. J. Cancer 79, 300–307.
6. Slamon, D. J., Godolphin, W., Jones, L. A., Holt, J. A., Wong, S. G., Keith,
D. E., Levin, W. J., Stuart, S. G., Udove, J., Ullrich, A., et al. (1989) Science
7. Borresen, A. L., Andersen, T. I., Eyfjord, J. E., Cornelis, R. S., Thorlacius, S.,
Borg, A., Johansson, U., Theillet, C., Scherneck, S. & Hartman, S. (1995) Genes
Chromosomes Cancer 14, 71–75.
8. Bergh, J., Norberg, T., Sjogren, S., Lindgren, A. & Holmberg, L. (1995) Nat.
Med. 1, 1029–1034.
9. Battaglia, F., Scambia, G., Rossi, S., Panici, P. B., Bellantone, R., Polizzi, G.,
Querzoli, P., Negrini, R., Iacobelli, S. & Crucitti, F. (1988) Eur. J. Cancer Clin.
Oncol. 24, 1685–1690.
10. Howat, J. M., Barnes, D. M., Harris, M. & Swindell, R. (1983) Br. J. Cancer 47,
11. Alizadeh, A. A., Eisen, M. B., Davis, R. E., Ma, C., Lossos, I. S., Rosenwald,
A., Boldrick, J. C., Sabet, H., Tran, T., Yu, X., et al. (2000) Nature (London)
12. Hedenfalk, I., Duggan, D., Chen, Y., Radmacher, M., Bittner, M., Simon, R.,
Meltzer, P., Gusterson, B., Esteller, M., Kallioniemi, O. P., et al. (2001) N. Engl.
J. Med. 344, 539–548.
13. Golub, T. R., Slonim, D. K., Tamayo, P., Huard, C., Gaasenbeek, M., Mesirov,
J. P., Coller, H., Loh, M. L., Downing, J. R., Caligiuri, M. A., et al. (1999)
Science 286, 531–537.
14. Perou, C. M., Sorlie, T., Eisen, M. B., van de Rijn, M., Jeffrey, S. S., Rees, C. A.,
Pollack, J. R., Ross, D. T., Johnsen, H., Akslen, L. A., et al. (2000) Nature
(London) 406, 747–752.
15. Geisler, S., Lonning, P. E., Aas, T., Johnsen, H., Fluge, O., Haugan, D. F.,
Lillehaug, J. R., Akslen, L. A. & Børresen-Dale, A.-L. (2001) Cancer Res. 61,
16. Tusher, V. G., Tibshirani, R. & Chu, G. (2001) Proc. Natl. Acad. Sci. USA 98,
5116–5121. (First Published April 17, 2001; 10.1073兾pnas.091062498)
17. Eisen, M. B., Spellman, P. T., Brown, P. O. & Botstein, D. (1998) Proc. Natl.
Acad. Sci. USA 95, 14863–14868.
18. Aas, T., Borresen, A. L., Geisler, S., Smith-Sorensen, B., Johnsen, H., Varhaug,
J. E., Akslen, L. A. & Lonning, P. E. (1996) Nat. Med. 2, 811–814.
19. Berns, E. M., Foekens, J. A., Vossen, R., Look, M. P., Devilee, P., Henzen-
Logmans, S. C., van Staveren, I. L., van Putten, W. L., Inganas, M., Meijer-van
Gelder, M. E., et al. (2000) Cancer Res. 60, 2155–2162.
20. Nakopoulou, L. L., Alexiadou, A., Theodoropoulos, G. E., Lazaris, A. C.,
Tzonou, A. & Keramopoulos, A. (1996) J. Pathol. 179, 31–38.
www.pnas.org兾cgi兾doi兾10.1073兾pnas.191367098 Sørlie et al.